Subterranean formations penetrated by an oil or gas well are often treated by hydraulically fracturing the formations to increase the production of oil or gas. Fracturing of the formations is accomplished by pumping fluids into the borehole of the well under high pressure so that cracks or fissures are opened into the surrounding formation. Typically, the fracturing fluid is a liquid that is thickened by addition of a polymer or a surfactant. The fracturing fluid's viscosity is proportionally related to the created fracture geometry and fracture width so that more viscous fluids will produce longer and wider fractures.
In certain formations aqueous acid solutions, such as hydrochloric acid, are used to improve the permeability of the formation, thereby increasing production. Acid can be used in either fracturing or matrix acidizing applications. Techniques of acid fracturing and matrix acidizing are well known in the art.
One technique used in acid fracturing consists of the use of a fluid which contains a viscosifying polymer and, optionally, a crosslinking agent. For instance, acids may be viscosified with natural polymers like xanthan, synthetic polymers like polyacrylamides. In addition to slowing down the reaction rate of the acid, such viscosifying agents further assist in controlling leak-off and acid diversion. Xanthans and polyacrylamides are typically employed as viscosifying agents in linear (non-crosslinked) acid based fluids. Crosslinked fluids usually consist of either a guar or guar derivative or a synthetic polymer base gel crosslinked with an organometallic compound. Crosslinked and linear gelled fluids may be energized, foamed with nitrogen or emulsified with carbon dioxide.
The synthetic polymers that are typically used as viscosifying agents in the aqueous acid solutions of the prior art are stable at the high temperature conditions which are commonly found in deeper oil and gas wells. While such polymers are capable of reducing the reaction rate of the acid, they are often unacceptable since the higher viscosity of the spent acid and the residue from the polymer may cause a significant amount of damage to the formation. The increased viscosity is attributable to the spending of the acid. In addition, the increased viscosity of the fluid makes clean-up of the well more difficult.
Another acid fracturing technique utilizes alternating stages of foamed or emulsified fluids and non-foamed or non-emulsified fluids. Benefits of staged acid fracturing treatments include increased fracture length, enhanced acid diversion, and enhanced cooling of the formation and inhibition or retardation of the reaction rate of the acid with the formation. Otherwise, conventional aqueous acid fluids often react too quickly, depleting the acid with very little penetration of the formation.
In addition to acid fracturing, aqueous acid solutions are further often used in matrix acidizing. Matrix acidizing is often used in sandstone formations to remove near wellbore damage. In matrix acidizing, a fluid containing an acid or acid-forming material is injected into the formation below fracture pressure such that the acid or acid-forming material reacts with minerals in the formation in the near wellbore. Permeability of the formation is thereby increased. Formation damage caused by drilling mud invasion and clay migration may also be removed during the process. Matrix acidizing treatments are intended to result in radial penetration and axial distribution of the acid. The quick reaction of the acid with the formation in the near wellbore, however does not allow for deep penetration into the formation.
Since polymeric viscosifying agents may cause damage to the formation, greater emphasis has been placed lately on viscoelastic surfactant based fluids. Exemplary viscoelastic surfactant based fluids reported in the literature include those set forth in U.S. patent application Ser. No. 10/345,104, filed on Jan. 15, 2003, now U.S. Patent Publication No. 20040138071, published on Jul. 15, 2004, herein incorporated by reference. Such fluids minimize damage to the formation and further are effective at high temperature conditions. These fluids further contain, however, inorganic acids which are damaging in hydrocarbon-bearing formations consisting principally of coal.
Further, the viscoelastic fluids of the prior art often have failed to penetrate areas distal to the wellbore. When used in carbonate formations, injection of the viscoelastic fluid within the wellbore often causes dissolution of calcium carbonate which, in turn, causes formation of a channel through the matrix. As additional fluid is pumped into the formation, the fluid tends to flow along the channel, leaving the rest of the formation untreated. With the acidizing fluids of the prior art, typically, an additional means of diversion, either mechanical or chemical, is used to get better distribution of the acid. Chemical diverters can include foams or viscous gels, injected as stages in between the acid stages. While improved viscoelastic fluids, such as those described in U.S. Patent Publication No. 20040138071, show improvements in matrix acidizing, such fluids are typically not useful in coal and/or shale formations.
A need therefore exists for the development of viscoelastic fluids which minimize damage to the formation, generates sufficient viscosity for diversion or fracturing applications and which react slowly with the near wellbore regions as the fluid is introduced into the formation. A need particularly exists for the development of such fluids for use in acid fracturing and matrix acidizing and especially for use in coal.